Anemia is a major source of morbidity and mortality worldwide, particularly among women and children. Much of this burden is attributable to defective red blood cell production (erythropoiesis). A greater understanding of erytkhropoiesis holds promise for the development of better therapies for various forms of anemia. Recent genome-wide association (GWA) studies have revealed over 75 loci associated with red blood cell traits. Furthermore, multiple bioinformatic analyses suggest that the majority of variation du to these common variants is intrinsic to red cell progenitors and precursors, allowing for directed studies on the effects of these genetic variants. Nevertheless, only a few studies of specific candidates have revealed new regulators of erythropoiesis. We and others have shown that many of the erythroid-trait associated loci (and variants in strong linkage disequilibrium (LD)) ar located in non-coding regions of the genome, enriched for empirically defined regions of open chromatin and erythroid enhancer elements marked by the erythroid transcription factors (TFs) GATA1, TAL1, KLF1, and NFE2. In select cases, we have shown that these variants are contained in and affect the activity of those enhancers and that they are dependent upon GATA1 activity. In this project, we propose to utilize a novel and innovate approach termed massively parallel reporter assay (MPRA) to systematically measure the activity of thousands of putative enhancer elements containing variants in strong LD with GWA study loci. Enhancers that show differential activity dependent upon the allele of the candidate variant contained will be prioritized for functional follow-up using cutting-edge CRISPR-Cas9 genome editing technology to confirm causality of that variant with respect to changes in proximal gene expression. Additionally, we will perform a second MPRA in GATA1-induced cells to examine whether each element is dependent upon the activity of the key hematopoietic TF GATA1. Increased differential enhancer activity will likely signal disruption of GATA1, co-factor, or nove TF binding sites, providing evidence of the underlying biological mechanism. Furthermore, each enhancer element of interest will be fully explicated per nucleotide using an innovative multiple mutagenesis approach in a similar MPRA, allowing for identification of TF binding site motifs linked to known or novel TFs involved in erythropoiesis. Successful execution of the proposed work will at a minimum identify multiple causal variants implicated in GWA studies of erythroid traits and create a public resource of erythroid-specific enhancer activity and important TF binding sites for future studies of erythropoiesis. Additionally, the proposed study establishes not just a new paradigm for high-throughput follow-up of GWA studies in troublesome non- coding regions of the genome, but creates a novel method for investigation of common transcriptional regulatory pathway disruption of cell-type or lineage-specific TFs and master regulators by common genetic variation which can be extended to many different cell types and human diseases.